The Prammer System allows communication and/or power transfer between surface equipment and downhole equipment and/or between different elements of downhole equipment. Downhole equipment may be located either close to a drill bit or anywhere else along a pipe string. The pipe string is comprised of many pipe joints that may be connected to each other via pin-and-box rotary connections. In an embodiment of the Prammer System, the box ends contain signal repeaters that compensate for the loss in signal amplitude along the cable segments spanning the pipe joints. Signals are communicated between pipe joints via electromagnetic resonance couplers that, being passive devices, also contribute to signal loss. The signal carrier frequency is typically located in the HF-to-UHF frequency range.
As described in the '384 patent, the electromagnetic resonance couplers comprise one or more antennas that may be printed on multi-layer laminates and may be arranged in a circular fashion, further comprising “capacitor blocks” that may be surface-mounted-devices (SMDs) embedded in the laminates. Generally, the purpose of these capacitors is many-fold. First, they cause electrical resonance by matching the physical length of the antenna segments to the electrical length required for resonance. Secondly, they match the high impedance of the resonant circuit to the much lower impedance of the attached one or more cable segments that typically have characteristic impedances of 50 Ohm or 75 Ohm.
The necessary presence of these capacitors complicates the manufacturing process and reduces the reliability of the couplers. The laminates may need to be hollowed out to house the capacitors, which may need to be glued in place and electrically connected, for example, by soldering and/or by means of electrically conductive glue. During operation, the capacitors are subjected to the full range of environmental conditions, including extreme downhole pressures and temperatures. It is unavoidable that mismatches exist between the thermal expansion characteristics of the ceramic material of the SMD capacitors and the thermal expansion characteristics of the laminate and also of the potting material, which may be epoxy-based or a thermoplastic such as polyaryletheretherketones (PEEK). These mismatches result in internal stresses, potentially leading to premature failures of the structures. It would be highly desirable to eliminate such thermal expansion mismatches.
The SMD capacitors also limit the power transmittable through a coupler. That power is limited by the voltage rating of the capacitors, which is typically 25 V or 50 V. The applied voltage must be further reduced if the capacitors are operated at the high end of their thermal ratings (typically 175° C. or 200° C.), which may be the case in a downhole environment. The maximum power P transferable through a coupler is given by P=sqrt((Z*V)/(sqrt(2)*Q)), where Z is the characteristic impedance of the cable, V is the peak voltage applied to the capacitors, and Q is the Q factor of the resonance. With typical values of Z=50 Ohm, V=25 V and Q=5, we find that the maximum transferable power is about 13 W. Given the temperature dependency, however, the maximum power should be kept below 10 W. It would be highly desirable to remove this power limitation and to be able to transfer power in excess of 10 W.
Thus, a need exists for a different coupler design, suitable for use in a wired-pipe downhole communications and/or power transmission system, without the need for discrete electronic components such as capacitors.